Controlling the gate of power semiconductor switches, such as insulated gate bipolar transistors (IGBTs) and metal oxide semiconductor field effect transistors (MOSFETs), is an important element of an inverter or a frequency converter. A power semiconductor switch in an inverter or a frequency converter is often configured to be controlled to one of two operational states: a conductive state (e.g., an on-state) or a non-conductive state (e.g., off-state).
Many aspects may have to be taken into account in the design of a driver circuit controlling the operational state of a power semiconductor switch. Operating voltage potential of a power semiconductor switch may radically change during operation, and the power semiconductor may operate at different voltage potential from the voltage potential the controller controlling the whole system. Therefore, a driver unit controlling the power semiconductor switch may be galvanically isolated from the controller.
The controller and the driver unit may be isolated from each other by an optical isolator, for example. An isolating power supply may be used for generating a positive voltage or a positive voltage and a negative voltage in order to be able to drive the semiconductor switch to a desired operational state.
For example, the semiconductor switch may be driven to the conductive state by supplying the positive control voltage to a control terminal, e.g. the gate terminal, of the semiconductor switch. The negative voltage may be used for producing a sufficiently high gate current during turn-off events and for ensuring that the semiconductor switch remains in the off-state even if voltage spikes appear in the gate voltage. In IGBTs, the supplied voltage/voltages is/are tied to a voltage potential of an emitter- or auxiliary emitter of the IGBT.
In order to achieve higher power ratings, parallel-connected power semiconductor switches may be used in inverters and frequency converters. However, because of physical differences between the parallel-connected switches and/or between the components implementing the driver circuitries controlling the switches, the switches may not turn on (or off) simultaneously. These non-concurrent switching events may cause additional losses.
Thus, it may be desirable to adjust the instants of turn-on and turn-off switching events of individual switches in order to achieve concurrent switching events. Various types of feedback implementations may be used for determining the instants of the actual switching event. For example, if the power semiconductor switch, e.g. an IGBT, is provided with a main emitter terminal and auxiliary emitter terminal, a voltage across a bonding wire between these terminals may be used for estimating the rate of change of emitter current. The rate of change may then be used for detecting a turn-on or turn-off event.
However, since the rates of change to be detected may be very fast, the measurement circuitry for detecting rates of change may have to be built from very high speed components. Further, in order to obtain measurement results from different switches in a manner that the results are accurately comparable, propagation delay skew of the detection circuitry may also have to be very low, in the order of magnitude of nanoseconds. An implementation able to meet these specifications may call for costly components and/or manufacturing processes, which may reduce cost-effectiveness of the implementation.
Further, the number of isolation channels for the control and/or feedback signal may have a significant effect on the cost-effectiveness of the system, for example, if modern high-temperature-rated CMOS digital isolators are being used. The availability of components may also have to be considered.
A three-phase frequency converter with a DC intermediate circuit can specify at least six power semiconductors for the output. Therefore, any increase or decrease in cost-effectiveness of a driver unit may have a six-fold effect on the frequency converter.